In general, construction machinery includes a hydraulic system. The hydraulic system receives power from an engine. The hydraulic system includes a hydraulic pump, a main control valve, an actuator, and an operating unit (a joystick and the like).
The hydraulic pump is driven by power of the engine to discharge working oil in which pressure is formed. The main control valve distributes and provides working oil to a desired actuator among a plurality of actuators. The actuator performs a desired operation by operating a corresponding working device by using working oil.
The engine generates power while consuming fuel. Torque of the engine implemented at specific engine speed (rpm) is changed. This will be described with reference to FIG. 1.
As illustrated in FIG. 1, when an engine speed is excessively high or low, torque of the engine is rather decreased, which may deteriorate energy efficiency. Further, when an engine speed is high, fuel consumption is increased. That is, the engine needs to be operated at an appropriate engine speed considering energy efficiency, and in this case, fuel efficiency may increase.
A rated engine speed is suggested to the engine. When an engine speed is lower than the rated engine speed, actually implemented torque is low, so that an engine stall phenomenon may occur when a larger load than the torque generated in the engine is applied. Particularly, when a large load is suddenly applied to the hydraulic system, the engine speed may sharply drop.
A load applied to the hydraulic system is increased/decreased in proportion to an operation displacement of the operating unit. Examples of the operating unit include a joystick and a pedal. Hereinafter, the operating unit will be described based on a joystick as an example.
A sharp operation of the joystick by an operator means a sharp increase of required torque. The increase of torque means that a discharged flow rate of working oil is increased or pressure of working oil is increased. In order to increase torque, when a discharged flow rate of working oil is uniformly maintained, pressure of the working oil needs to be increased. The increase of the pressure of the working oil means that a load is applied to the hydraulic pump, and the load applied to the hydraulic pump is applied to the engine as a load.
Horsepower control by a negative control manner (or referred to as “negacon”) known in the related art changes secondary pressure of a pilot pump by adjusting a current value supplied to an electronic proportional pressure reducing (EPPR) valve attached to the pump, and controls horsepower set in the pump. To this end, a vehicle control device determines a current value by proportional derivative (PD) control or offset control so that a fixed rated engine speed is maintained by setting an engine speed by a dial in advance.
Further, the rated engine speed is to set to a level of 100 rpm in a high idle state so that the pump may use maximum horsepower. The rated engine speed is designed so as to maximally utilize horsepower of the pump, but there are many cases where it is impossible to use full horsepower by the dynamic characteristic of the engine, so that tuning of the rated engine speed is limited. Further, fuel is unnecessarily consumed and exhaust gas and the like are generated due to a gap between an engine speed when the operator sharply operates the joystick and the rated engine speed.
A problem occurring when the operator sharply operates the joystick will be described with reference to FIG. 2. As illustrated in FIG. 2, the joystick is sharply operated at a specific moment, so that an engine speed is sharply decreased from a time point t1 requiring large torque, and the engine speed is decreased to be lower than a rated engine speed at a specific moment t2. Then, a turbocharger is operated, and a time is taken until the turbocharger exhibits a normal function. As described, when the function of the turbocharger is normally performed, the engine speed is gradually restored.
The engine speed may be more severely decreased when the dynamic characteristic of the engine is changed, and even in this case, the engine consumes the larger amount of fuel in order to implement required torque. That is, the consumption of the large amount of fuel means that fuel efficiency extremely deteriorates, and causes exhaust gas.
Particularly, the hydraulic system known in the related art corrects a rated engine speed by setting a dynamic characteristic and a characteristic of a torque curve of the engine by constants and collectively reflecting the constants, but there is a limit in correction work, and thus there is a problem in that the dynamic characteristic of the engine fails to be properly reflected.